The present invention relates to a method and to a sensor system for measuring the transmission properties of a first transmission path between a first transmitter and a receiver. The first transmitter transmits a first signal onto the transmission path that is detected by the receiver after passing through at least a portion of the first transmission path. A second transmitter transmits a second signal onto a second transmission path that is detected by the receiver after passing through the second transmission path. In the receiver, the first signal and the second signal are superimposed in a substantially cumulative manner. A receiver output signal is then formed therefrom, processed further, and finally fed to the second transmitter in a feedback loop for compensation.
In many areas of application, the transmission properties of a transmission path from a transmitter to a receiver, for example a sensor, are to be determined. It can be necessary, for example, to determine the distance of a reference object to another object or to detect the movement of an object in a certain area or space. Other examples are the determination of fundamental properties of the medium of the transmission path such as, for example, refractive index, absorption spectra, dispersion, scattering (e.g., fog detection), spin relaxation times, electromagnetic constants such as permittivity, relative permeability and light velocity, fluorescence characteristics (transmission of carrier wavelength and/or carrier frequency of a signal to another wavelength/frequency) or the detection of the properties of boundary layers and surfaces such as reflectivity, reflection spectrum, ratio of the abovementioned properties between the media of the boundary surface, spacing of the boundary surface, etc.
Numerous compensating methods are known from the prior art in which the actual transmission signal is superimposed with a compensating signal at the receiver in such a way that the receiver contains a nearly constant signal in sum. Such a compensating method of measurement is disclosed, for example, in the following documents:
DE 10001955A1DE 10024156A1DE 19839730C1DE 930983U1DE 10001943C1DE 10346741B3DE 102004025345B3DE 102005013325A1DE 102005010745B3DE 102007005187B4
The methods described in these documents use an amplitude-regulated analog signal as a transmission signal and/or as a compensation signal. The transmission signal has a constant duty cycle and as a rule is substantially monofrequent, that is, it uses only one specific frequency.
The monofrequent, analog compensation signal turns out to be a substantial drawback in these methods, because the monofrequent compensation signal is very susceptible to interference. For example, it has been found that, in optical systems, while there is no interference from more or less uniform sunlight, light from light sources such as fluorescent tubes and transients thereof that fluctuates in multiples of the line frequency of 50 Hz or 60 Hz of the respective regional line frequency does indeed cause interference. The problem is exacerbated all the more when several of the measuring systems described therein cannot be optically separated. This can be the case, for example, in an optical automobile radar based on one of the above disclosures if the transmitter of an oncoming automobile transmits into the receiver of one's own automobile. The above systems are not usable for this purpose.
The systems are therefore not readily suitable for triangulation and more complex recognition tasks using several systems.
It also becomes difficult to use such systems if several transmission paths or a larger range are to be detected or monitored. A time division multiplex method is then generally used in order to differentiate the individual signals in time. However, this has the drawback that delays occur during the processing of the signals and observation of the desired range. Time-gaps arise in the spatial observation that are not acceptable, particularly in security-related applications.
Another drawback of the systems from the above list that perform gap detection is that they are only capable of determining the average gap between the reflectors when several reflectors are used.